This invention relates in general to communication systems, and more specifically to a method and apparatus for creating a radio frequency filter.
Hardware suitable for a Software Defined Radio (SDR), in addition to its many other requirements, must be frequency agile. That is to say, in order to be truly useful, the RF section must be able to cover a wide bandwidth. However, it is insufficient to simply create a wideband, untuned RF section, since in order to meet the RF performance specifications of many services with practical components it is necessary to provide more narrowband band pass filtering, or at least low pass filtering, to reject image frequencies, spurious responses, blockers, and other undesirable signals in a receiver, and harmonics, spurious responses, far-out noise, and other undesirable signals in a transmitter. For an SDR it is desirable that band pass RF filtering be adjustable in both center frequency and bandwidth, to provide the greatest flexibility. Similarly, if low pass filtering is employed, it is desirable that the low pass filtering have a selectable corner frequency. In addition, users are now enjoying the low power consumption and excellent dynamic range of fixed, passive RF selectivity in their single-mode radios. A successful SDR should have similar performance to be successful in the marketplace.
No truly satisfactory solution to this requirement exists in the prior art. One prior-art SDR design incorporates a large number of switchable, passive band pass RF filters, each of which may be individually varactor tuned. While this brute-force approach works, it is not a technology transferable to small, low-cost portable equipment. Many types of active filtering have been suggested for the SDR application, from gmC filters to logarithmic filtering, but they all suffer from dynamic range and current drain limitations when compared to the passive filtering used in existing products.
What is needed is a method and apparatus for creating a multi-band RF filter that is flexible in both center frequency and bandwidth, and that maintains the current drain and dynamic range performance of passive RF filters.